How to assign different materials to different colors in a texture?
You can make a mask texture. This kind of map is black and white, where black means 0, and white means 1. So if you plug it into a Mix Shader node with your blood and steel materials like so:

The black areas of the mask texture will indicate steel material, and white will indicate the blood material. So paint the steel portion of your mesh black and paint the blood portion white, and they will have separate materials.